INTRODUCTION
The relative humidity inside of a home or building, referred to as the indoor RH, should be maintained below certain levels for the comfort and health of the occupants as well as for the preservation of the building and its contents.
The indoor RH that is comfortable for humans varies from individual to individual and is dependent on temperature and air currents (from fans and furnace blower). For human comfort, during the cooling season, the indoor RH should not exceed 60%. Annoying insects such as earwigs and millipedes in a home can be reduced or eliminated by reducing the indoor RH with a dehumidifier.
High indoor RH is associated with numerous health issues for the occupants. These include allergies, asthma, arthritis, gout, infections and diseases from fungi and fungal toxins and many others. Based on health issues it is recommended that the indoor RH be maintained below 60%.
High indoor RH increases the probability that the home or building and its contents may be damaged. Corrosion, warping of wood, fungal (mold) growth and damage from insects will occur if the indoor RH is high enough. Damage to or malfunction of the building structure, electronics (TV, computers, etc.), medical devices (Eye Lasik, MRI scanner, etc.) furniture (some may be antiques), other wood items used in the construction of the home or building like floors and doors, musical instruments (piano, violin, guitars, etc.), wall coatings (paint and wall paper), paper items (books, periodicals, comics, stamps, art, etc.) may occur and occur more rapidly with increasing RH. For preservation of the building and its contents it is recommended that indoor RH be maintained below 60%.
Therefore, based on comfort and health of the occupants as well as the preservation of the home or building and its contents, the indoor RH should be maintained below 60%. Throughout this paper 60% will be used as the indoor RH by which comfort, health and preservation of the home and it contents will be judged.
Conventional air conditioning, the type of air conditioning installed in most every home, cools the air as well as removing moisture from the air and is the most frequently used equipment to reduce the indoor RH. Conventional air conditioning does not provide control of indoor RH and does not maintain the indoor RH at acceptable levels throughout the year because it operates based on indoor temperature (sensible cooling load) and not on indoor RH (latent load). In northern climates, there is a small sensible cooling load during the months of May & June, September & October and at nights during the summer months (July & August). In southern climates there is a small sensible cooling load from December through March. Therefore, these months will present the most challenging times in maintaining acceptable indoor RH level at these locations.
If a dehumidifier is used in conjunction with air conditioning and operated properly, the indoor RH can be maintained below 60% RH, while providing a significant reduction in energy usage (energy savings). The purpose of this paper is to present some operational strategies for air conditioning and dehumidification systems that provide acceptable indoor RH levels and significant energy savings.
METHODS
A residential home in Miami, FL was simulated on an hourly basis with TRNSYS, a widely used building simulation program. The home is referred to as a "High Efficiency House" and is designed for a hot, humid climate. This home is representative of a newly constructed air tight, energy efficient home as well as a condominium.
The home has a slab-on-grade foundation, which is the predominant foundation type in a hot humid climate. The home has 4 bedrooms, 8.0 foot high ceilings and 2000 ft² of conditioned floor area. The home is extremely energy efficient and air tight. R-values for the walls, ceiling and floor are 19.4, 38.5, and 13.0 (hr-ft²-°F)/Btu respectively. The median infiltration rate, based on a variable infiltration model, is 0.17 ach. There is no supplemental mechanical ventilation.
Peak and average occupancy rate for the home is 3.0 and 2.4 people. The Internal Moisture Gain from human occupation is 10.3 pounds daily and is on a fixed schedule. An additional, variable amount of moisture that contributes to the Internal Moisture Gain is included in the model and is from re-evaporation off the air conditioning coil. Moisture capacitance of building materials and the contents of the home is included in the model.
The capacity of the air conditioner in the home is 1.7 tons with an Energy Efficiency Ratio, EER, of 13.3 (Btu/hr)/watt without the HVAC fan and a Seasonal Energy Efficiency Ratio, SEER, of 13 (Btu/hr)/watt. The same air conditioner is used in the conventional air conditioning system and in the air conditioning and dehumidification system. The small size of the air conditioner, 1.7 tons, is a result of the home being extremely energy efficient and air tight. The HVAC system fan uses 0.18 watts/cfm, delivers 680 cfm and operates only when cooling is required. The dehumidifier has an integral fan and operates only when the indoor RH exceeds the RH set point and has a rated capacity of 90 pints/day. Internal sensible heat gains include fixed schedule lighting and other equipment in the home (5,635 kWh annually), as well as an amount from the dehumidifier that depends on indoor conditions.
RESULTS
STRATEGY 1: Cooling Set Point
A number of factors affect the thermal comfort of the occupants of a home or building and include air temperature, air currents and humidity. Occupants experience equivalent comfort at higher temperatures when the indoor RH is lowered. ASHRAE incorporates this concept in the comfort zone in ASHRAE Standard 55-2004, Thermal Environmental Conditions for Human Occupancy. A published report indicates that schools served by a dehumidification system were maintained warmer than schools served by a conventional air conditioning system with equivalent occupant comfort.
In this study, the performance of a conventional air conditioning system with a temperature setting (set point) of 75.0 °F was compared to an air conditioning and dehumidification system with set points of 78.0 °F and 59% RH. The difference in the cooling set point in this study is 3.0 °F. The 59% set point for the dehumidifier was chosen to keep the indoor RH below 60%.
Figure 1: Indoor RH vs Hour of Year for Strategy 1
A line graph showing Indoor RH % on the Y-axis against Hour of Year on the X-axis. Two lines are plotted: a blue line for the AC System, showing significant periods above 60% RH (totaling 1,641 hours or 18.7% of the year), and a pink line for the AC & DH System, which remains consistently below 60% RH (0 hours above 60%). The legend indicates "Annual Hours > 60% RH".
Figure 2: Total Energy Usage vs Month for Strategy 1
A bar chart showing Total Energy Usage (kW) on the Y-axis against Month on the X-axis. Two sets of bars are shown for each month, representing the AC System (dark blue) and the AC & DH System (purple). The AC & DH System consistently shows lower energy usage than the AC System, with notable savings from April through November. The legend indicates total annual usage for AC System (3,417 kW) and AC & DH System (2,808 kW).
Table 1: Annual Performance Summary for Strategy 1
| Performance Parameter | Air Conditioning & Dehumidifier | Conventional Air Conditioning |
|---|---|---|
| Energy Usage, kWh | 2,808 | 3,417 |
| % Energy Savings | 18 | NA |
| Operation Time, hours | 1279 (AC) 258 (DH) | 1911 |
| Time > 60% RH, hours | 0 | 1,641 |
| % Time > 60% RH | 0 | 18.7 |
STRATEGY 2: Overcooling
One practice that is often employed to improve the comfort level in a home or building is to significantly lower the temperature setting (set point) for the conventional air conditioning system. This is referred to as overcooling. Overcooling improves comfort by reducing both the indoor temperature and the indoor RH. Overcooling is frequently employed even though it consumes a significant amount of energy and may result in moisture and mold problems if the cooling set point is lower than the dew point of the outdoor air.
In this study, the performance of a conventional air conditioning system operating at a set point of 72.0 °F (overcooling) was compared to an air conditioning and dehumidification system with set points of 78.0 °F and 59% (the same system from Strategy 1).
Figure 3: Indoor RH vs Hour of Year for Strategy 2
A line graph showing Indoor RH % on the Y-axis against Hour of Year on the X-axis. Two lines are plotted: a blue line for the AC System, showing periods above 60% RH (totaling 280 hours or 3.2% of the year), and a pink line for the AC & DH System, which remains consistently below 60% RH (0 hours above 60%). The legend indicates "Annual Hours > 60% RH".
Figure 4: Total Energy Usage vs. Month for Strategy 2
A bar chart showing Total Energy Usage (kW) on the Y-axis against Month on the X-axis. Two sets of bars are shown for each month, representing the AC System (dark blue) and the AC & DH System (purple). The AC & DH System consistently shows significantly lower energy usage than the AC System, with savings occurring every month. The legend indicates total annual usage for AC System (4,973 kW) and AC & DH System (2,808 kW).
Table 2: Annual Performance Summary for Strategy 2
| Performance Parameter | Air Conditioning & Dehumidifier | Conventional Air Conditioning |
|---|---|---|
| Energy Usage, kWh | 2,808 | 4,973 |
| % Energy Savings | 44 | NA |
| Operation Time, hours | 1279 (AC) 258 (DH) | 2757 |
| Time > 60% RH, hours | 0 | 280 |
| % Time > 60% RH | 0 | 3.2 |
STRATEGY 3: Unoccupied in Summer Months
In the US, about one in four retiree "snowbirds" (about 900,000 people) reside in Florida to escape cold, northern climates during the winter. This seasonal occupancy creates a need for strategies for controlling the indoor RH to avoid mold problems while minimizing energy usage when the home is unoccupied. Previous studies have indicated that raising the set point of a conventional air conditioning system to 83.0 to 85.0 °F during the unoccupied period reduces energy usage but does not provide sufficient indoor RH control.
In this study, the performance of a conventional air conditioning system with a set point of 78.0 °F throughout the year was compared to an air conditioning and dehumidification system with set points of 78.0 °F when occupied (November through April), and 85.0 °F when unoccupied (May through October) and 59% RH year-round.
Figure 5: Indoor RH vs Hour of Year for Strategy 3
A line graph showing Indoor RH % on the Y-axis against Hour of Year on the X-axis. Two lines are plotted: a blue line for the AC System, showing significant periods above 60% RH (totaling 2,581 hours or 29.5% of the year), and a pink line for the AC & DH System, which remains consistently below 60% RH (0 hours above 60%). The legend indicates "Annual Hours > 60% RH".
Figure 6: Total Energy Usage vs Month for Strategy 3
A bar chart showing Total Energy Usage (kW) on the Y-axis against Month on the X-axis. Two sets of bars are shown for each month, representing the AC System (dark blue) and the AC & DH System (purple). The AC & DH System consistently shows significantly lower energy usage than the AC System, with savings occurring during the unoccupied period from May through October. The legend indicates total annual usage for AC System (2,525 kW) and AC & DH System (1,447 kW).
Table 3: Annual Performance Summary for Strategy 3
| Performance Parameter | Air Conditioning & Dehumidifier | Conventional Air Conditioning |
|---|---|---|
| Energy Usage, kWh | 1,447 | 2,525 |
| % Energy Savings | 43 | NA |
| Operation Time, hours | 414 (AC) 354 (DH) | 1245 |
| Time > 60% RH, hours | 0 | 2,581 |
| % Time > 60% RH | 0 | 29.5 |
DISCUSSION
This study clearly indicates that there are operational strategies in which an air conditioning and dehumidification system can be used to provide significant energy savings as compared to a conventional air conditioning system, while significantly improving comfort and health of the occupants as well as preservation of the building and its contents.
Similar results would be found in other humid locations along the gulf coast and in the south such as Houston, TX and Jacksonville, FL since the cooling and dehumidification loads are similar. Increasing the air exchange rate from ventilation or looser construction increases the number of hours the indoor RH is greater than 60% with the conventional air conditioning system, making a dehumidifier even more beneficial. Simulations done in less humid Midwestern locations indicate an air conditioning and dehumidification system can provide better comfort with similar energy usage and in some cases an energy savings. Other energy and cost saving strategies for the air conditioning and dehumidification system could be employed when the home is unoccupied such as running the dehumidifier during off peak hours. The best time of day for the dehumidifier to run is based on the size and air tightness of the home.
